Thin-film transistors, which are also referred to as TFT, are realized in polycrystalline or amorphous semiconductor layers. These semiconductor layers are produced, for example, by deposition of polysilicon, amorphous silicon, Si--C or diamond and, if necessary, by subsequent recrystallization. The semiconductor layer is thereby applied on a substrate.
Thin-film transistors are realized in planar form (see, for example U.S. Pat. No. 5,064,775). In order to avoid a short between a source region and a drain region via the substrate, the substrate must therefore be insulating at least at the boundary surface to the polycrystalline semiconductor layer. Insulating substrates of glass, oxides or nitrides are preferably employed for thin-film transistors,
When a planar thin-film transistor is to be fabricated on an electrically-conductive foundation, then an electrically-insulating intermediate layer must be deposited between the foundation and the polycrystalline semiconductor layer. Charges captured at the boundary surface between the polycrystalline semiconductor layer and the insulating intermediate layer or interconnects located in the foundation lead to electrical fields that have a disadvantageous effects on the charge transport in the thin-film transistor.
U. Mitra et al, J. Electrochem. Soc. 138, page 3420, (1991), discloses that disturbing boundary surface charges in oxides be reduced by tempering in forming gas.
N. Ibaraki et al, ED36, page 2971, 1989, has proposed that the insulating intermediate layer be fashioned of a different material, for example nitride, that has more beneficial boundary surface properties. The plurality of disturbing boundary surface charges is to be reduced in this way by selecting the material of the intermediate layer.
Electrical fields elicited by an interconnect under the thin-film transistor, these electrical fields deteriorating the functioning of the thin-film transistor, can be shielded by inserting an additional shielding layer (see Nakashima et al, Electronics Lett. 19, page 1095 (1983)).
Polycrystalline semiconductor layers have grain boundaries. In the thin-film transistor, these grain boundaries lead to disturbances of the charge transport in the MOS channel. It is known (see H. N. Chern et al., EDL 14, page 115 (1993)) to reduce the disturbances of the charge transport at the grain boundaries by passivation of imperfections. To that end, a polycrystalline silicon layer is tempered in a hydrogen atmosphere or in a plasma that contains hydrogen and/or oxygen.
The properties of the thin-film transistors are deteriorated due to the capture of charge carriers and scatter of charge carriers at grain boundaries. By comparison to MOS transistors integrated in monocrystalline silicon, thin-film transistors currently exhibit a current yield that is reduced by a factor of 20-100. The low current yield of thin-film transistors must be compensated by larger geometrical dimensions. This leads to an increased use of area.
A. O. Adam et al., VLSI Symp. page 19 (1990), has proposed that the current yield of a thin-film transistor be increased by attaching an additional control electrode to the underside of the thin-film transistor. However, this increases the complexity of the component.